It is a known fact that communication participants exchange data with one another in compliance with a communication protocol. An arrangement for performing a data exchange in this manner is shown in FIG. 1. Communication participants A and B communicate with one another via a communication medium (CM) such as an electrical line by exchanging messages or protocol data units (PDUs) in accordance with a communication protocol. The communication protocol constitutes a complete set of rules for the required behaviour of each participant A and B in the communication. Participants A and B are instances in the sense of the OSI reference model which is described in more detail in xe2x80x9cISO. Information Processing Systemsxe2x80x94Open Systems Interconnectionxe2x80x94Basis Reference Modelxe2x80x9d International Standard ISO/IS 7498, ISO, 1984.
It is the object of the present invention to capture the timing of communication states of the one or the other participant in the communication which occur according to the communication protocol and to detect violations of the communication protocol as they occur.
For accomplishing this object, a method for checking a data exchange between communication participants via a communication medium in accordance with a communication protocol, said method being defined based on the extended finite state automaton principle, captures the data exchange by means of a protocol monitor connected to the communication medium and containing a test automaton which latter is likewise defined based on the extended finite state automaton principle. Said test automaton contains the same states and state variables as the protocol automaton defining the communication protocol, apart from the fact that in the test automaton, one value of the state variable is allocated a plurality of values from the range of values of corresponding state variables of the protocol automaton. Based on a state of the test automaton which comprises all states and all values of the state variables of the protocol automaton, transitions of the protocol automaton as observed on the communication medium are speculatively simulated in the test automaton and, by a process of logical elimination, a state is shaped in the test automaton which matches the relevant state in the protocol automaton.
An essential advantage of the method according to the invention is that it allows the data exchange to be checked at any random start, i.e. without knowledge of the current state of the communication, since the protocol monitor is capable of concluding the state of the communication participants from the data captured on the communication medium.
Frequently the communication exchange between communication participants takes place on the basis of communication protocols which are defined on the principle of a plurality of cooperating extended finite state automatons. In such a case, in order to gain insight into the communication state without excessive computational work and in relatively short time, if the inventive method is applied in a data exchange between communication participants in compliance with a communication protocol, said method being defined on the principle of a plurality of cooperating extended finite automatons, the protocol monitor contains test automatons in a number corresponding to the number of the extended finite state automatons defining the communication protocol, with each test automaton being defined by states and state variables corresponding to the relevant allocated automaton of the communication protocol.